Communication systems that geographically reuse communication resources are known. These systems allocate a predetermined set of communication resources in one geographic area and reuse the same set of communication resources in one or more other geographic areas. This reuse technique improves communication system capacity by minimizing the number of communication resources necessary to provide communication service in a large geographic area comprised of several smaller geographic areas. As is also known, communication resources are defined by the multiplexing scheme utilized in the particular communication system. For example, with frequency division multiplexing (FDM), a communication resource may be a frequency carrier or pair of frequency carriers; whereas, with time division multiplexing (TDM), a communication resource may be a time slot, or pair of time slots, in one or more time frames.
In geographic reuse communication systems, signal usability of a communication resource needs to be determined in order to reliably use the communication resource. The signal usability of a communication resource provides an indication of the corruption on that communication resource due to interference and noise present on a radio frequency (RF) channel. The RF channel provides a medium through which the communication resource is transceived by a communication unit or base station. Thus, signal usability is typically limited by the quantity of co-channel interference and noise present on the RF channel. Co-channel interference occurs when receivers receive unwanted information signals from neighboring communication units, or base stations, transmitting on the same channel as the desired RF channel. Noise occurs due to various phenomena, such as thermal noise from resistors within a receiver, shot noise from automobile alternators, and background noise from atmospheric sources.
Another alteration of a transmitted signal occurs as a result of fading. Fading occurs due to multiple reflections of the modulated signal during transmission over the RF channel. These reflections typically result from unintentional reflecting of the modulated signal from obstacles in its path, such as walls, automobiles, and buildings, and may produce multiple modified replications of the modulated signal, each introducing various amplitude and phase alterations of the original signal in each new signal path. All of the modulated signal replicas form a composite signal at the input to a receiver and account for the fading.
In order to mitigate the effects of fading, radio communication systems typically utilize diversity to enhance the signal-to-noise ratio of the modulated signal in a fading environment. Diversity techniques are incorporated in communication receivers and attempt to obtain multiple, decorrelated replicas of the transmitted signal by either using multiple antennas typically spaced several wavelengths apart or receiving redundant transmissions at predetermined time intervals. Thus, by receiving multiple copies of the transmitted signal, the diversity receiver produces an output signal with a better overall signal-to-noise ratio than if only one copy of the transmitted signal were received.
Although a variety of diversity techniques exist, two of the most common types are switched-branch diversity and time diversity. Switched-branch diversity is a technique typically used in a communication unit's receiver, wherein two receive antennas are separated in space by at least one quarter of a wavelength and are used to receive decorrelated replicas of a transmitted signal. An antenna switch in the communication unit enables the receiver to sequentially sample the signal received from each antenna. Each sampled signal is analyzed by the receiver to determine which is more preferable. Upon selecting the more preferable signal, the receiver directs the antenna switch to access the antenna which provided the more preferable signal. The receiver then continues to receive the transmitted signal using the selected antenna. The signal received from both antennas is periodically sampled and analyzed to insure continued use of the better signal.
A common method for choosing the preferred signal is to measure a received signal strength indication (RSSI) of the signal received from each antenna. The RSSI contains the sum of the transmitted signal and the co-channel interference plus noise on the RF channel over which the transmitted signal propagated. With the RSSI method, the antenna that produces the signal with the larger RSSI is subsequently selected. Since the RSSI comprises a summation of the desired signal and interference, a large RSSI may be obtained when an excessive level of co-channel interference and noise exists on the RF channel as compared to the level of the originally transmitted signal. In this case, the usability of the received signal is poor although the RSSI is large. Therefore, an RSSI fails to provide an accurate indication of a received signal's usability since it does not isolate the desired signal from the co-channel interference and noise. Further, the RSSI may be significantly impacted by the gain of each receiving antenna. Thus, the larger RSSI may be produced from the signal received by the higher gain antenna although it may not necessarily be the signal having the better signal usability.
The second diversity technique, time diversity, may be used by a receiver in either a communication unit or a base station. Time diversity only necessitates one antenna, but requires the same information to be transmitted at two different times. The receiver independently receives each transmission and determines respective RSSIs. Each signal is subsequently weighted based on its respective RSSI, with a higher weighting given to the signal with the larger RSSI. The weighting may include attenuating, or even eliminating, one of the received signals. The two weighted signals are then combined to provide a composite signal with an improved overall signal-to-noise ratio. However, by utilizing RSSI to weight the two received signals, a signal with an excessive level of co-channel interference and noise may be weighted higher and subsequently degrade the composite signal's signal-to-noise ratio.
Therefore, a need exists for a method and apparatus that produce a usable signal from modulated signals received by a diversity receiver based on an estimation of RF channel interference. Further, a diversity system that does not rely on an RSSI would be an improvement over the prior art.